I think it's not really another person's business what another person does in his/her private life unless they're in a relationship together, and/or it's clearly unhealthy for one or both parties but for some reason one or both are oblivious or in denial. Even then, you can only tell someone your feelings, not force them to do or not do something. I think that in her shoes, I would have broken it off with him because I'd rather not be in a relationship with someone who has a lot of partners (if that's their preference, cool. It's not mine, though), but would have tried to help him if he was unhappy with his situation, even if that help meant being a shoulder to cry on. Also, the beating up doesn't sound cool or warranted. If it was used as a tool for character growth and everyone got punished for bad behavior and learned something, okay, but that sounds ham-fisted. And okay, the edit changes some things. I can see her getting upset that 'not exclusive' turned into 'I had sex with fifty women in the past two weeks', but you don't condone someone beating that person up. No.

Also, Fifty Shades made me want to punch a koala.